Given the generally agreeable temperatures and dismayingly frequent quasi-monsoon downpours, this past summer often didn't feel like a real summer at all. It may have passed us by entirely, in fact, if not for the River to River Festival's annual habit of throwing ambitious and wildly eclectic artistic fetes (featuring Conor Oberst, or Quimbombó, or the New York City Opera, etc.) at various, oft-outdoor joints around town, beginning in late May/early June with the biggest, weirdest, most delightful event of all: the Bang on a Can Marathon. A two-decade-plus institution lately spanning 12 hours (it used to reach past 24) and currently held in the World Financial Center's gorgeous, monolithic, pleasantly airport-like Winter Garden, the free Marathon serves up an ambitious lineup of composers, new-music world premieres, and just a few indie-rock ringers. With the Bang on a Can crew at the helm, it's easily the most challenging popular musical event of the year: Folks just camp out on the Garden's elegant, uh, marble and luxuriate in the dulcet sounds of Steve Reich, Ryuichi Sakamoto, Terry Riley, and so forth. You always emerge bleary-eyed but spiritually/musically refreshed, and boom: It's summer.